Exposure apparatus including silica glass and method for producing silica glass

ABSTRACT

A silica glass has a structure determination temperature of 1200 K or lower and an OH group concentration of at least 1,000 ppm. The silica glass is used for photolithography together with light in a wavelength region of 400 nm or shorter.

This application is a divisional application of U.S. patent applicationSer. No. 08/581,017, filed Jan. 3, 1996, now U.S. Pat. No. 6,087,283.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silica glass for photolithography,optical members including the glass, an exposure apparatus including thesame, and a method for producing the glass. More particularly, itrelates to a silica glass used in photolithography techniques togetherwith light in a wavelength region of 400 nm or shorter or, morepreferably, 300 nm or shorter, optical members such as lens and mirrorincluding the glass, an exposure apparatus including the glass, and amethod for producing the glass.

2. Related Background Art

In recent years, VLSI has been produced with a higher integration and ahigher functionality. Particularly, in the field of logical VLSI, alarger system has been mounted on a chip, namely, system-on- chiptechnique has been in progress. In conjunction with such a trend, thereis a demand for finer processability and higher integration on a wafer,such as that made of silicon, which constitutes a substrate for VLSI. Inphotolithography techniques according to which fine patterns ofintegrated circuits are exposed to light and transferred onto waferssuch as of silicon, exposure apparatuses called stepper are used.

In the case of DRAM, as an example of VLSI, with the advance from LSI toVLSI, as its capacity gradually increases from 1 KB through 256 KB, 1MB, 4 MB, and 16 MB to 64 MB, the processing line width required for thestepper correspondingly becomes finer from 10 μm through 2 μm, 1 μm, 0.8μm, and 0.5 μm to 0.3 μm.

Accordingly, it is necessary for a projection lens of the stepper tohave a high resolution and a great depth of focus. The resolution andthe depth of focus are determined by the wavelength of the light usedfor exposure and the N.A. (numerical aperture) of the lens.

The angle of the diffracted light becomes greater as the pattern isfiner, whereas the diffracted light cannot be captured when the N.A. ofthe lens becomes greater. Also, the angle of the diffracted lightbecomes smaller in the same pattern as its exposure wavelength A isshorter, thereby allowing the N.A. to remain small.

The resolution and the depth of focus are expressed as indicated by thefollowing equations:

resolution=k1·λ/N.A.

depth of focus=k2·λ/N.A. ²

wherein k1 and k2 are constants of proportionality.

In order to improve the resolution, either the N.A. is increased or λ isshortened. However, as can be seen from the above equations, it isadvantageous, in terms of the depth of focus, to shorten λ. In view ofthese points of view, wavelength of light sources becomes shorter fromg-line (436 nm) to i-line (365 nm) and further to KrF excimer laser beam(248 nm) and ArF excimer laser beam (193 nm).

Also, since the optical system loaded in the stepper is constituted by acombination of numerous optical members such as lenses, even when eachlens sheet has a small transmission loss, such a loss is multiplied bythe number of the lens sheets used, thereby decreasing the amount oflight at the irradiated surface. Accordingly, it is necessary for theoptical member to have a high degree of transmittance.

Therefore, in the steppers using light in a wavelength region of 400 nmor shorter, optical glass made by a specific method in view of theshortening of wavelength as well as the transmission loss due to thecombination of the optical members is used. Also, in the steppers usinglight in a wavelength region of 300 nm or shorter, it has been proposedto use synthetic silica glass and a fluoride single crystal such as CaF,(fluorite).

As a specific method for measuring internal transmittance, for example,a method of measuring transmittance of optical glass is known from JOGIS17-1982. Here, the internal transmittance is calculated by the followingequation: $\begin{matrix}{{\log \quad \tau} = {{- \frac{{\log \quad {T1}} - {\log \quad {T2}}}{\Delta d}} \times 10}} & (1)\end{matrix}$

wherein τ is internal transmittance of the glass when its thickness is10 mm; d is difference in thickness of a sample; and T1 and T2 arespectral transmission factors of the glass having sample thicknessvalues of 3 mm and 10 mm, respectively, including their reflection loss.

SUMMARY OF THE INVENTION

However, the inventors have found that, in the optical members composedof the conventional silica glass whose internal transmittance is definedin this manner, although a certain magnitude of the resolution issecured in terms of their specification, contrast of an image resultingtherefrom may be so unfavorable that a sufficiently vivid image cannotbe obtained.

Here, the contrast is defined by the following equation: $\begin{matrix}{\text{contrast} = \frac{{I\quad \max} - {I\quad \min}}{{I\quad \max}\quad + {I\quad \min}}} & (2)\end{matrix}$

wherein Imax is maximum value of optical intensity on a wafer surfaceand Imin is minimum value of the optical intensity on the wafer surface.

The object of the invention is to provide a silica glass forphotolithography which can overcome the foregoing shortcomings of theprior art and can realize a sufficiently fine and vivid exposure andtransfer pattern with a favorable contrast.

Accordingly, the inventors have studied, among the transmission lossfactors in the silica glass (optical member) used for photolithographytechniques and the like, factors for decreasing the contrast of image.As a result, it has been found that not only the optical absorption atthe silica glass but also its optical scattering causes the transmissionloss and that the amount of loss in light based on such opticalscattering (scattering loss amount) can be sufficiently suppressed whenthe structure determination temperature in the silica glass containingat least a predetermined amount of OH group is reduced at least to apredetermined level. Thus, the present invention has been accomplished.

The silica glass (fused silica, quartz glass) of the present inventionis used for photolithography together with light in a wavelength regionof 400 nm or shorter and is characterized in that it has a structuredetermination temperature of 1,200 K or lower and an OH groupconcentration of at least 1,000 ppm.

Further, the optical member (optical component) of the present inventionis an optical member used for photolithography together with light in awavelength region of 400 nm or shorter and is characterized in that itincludes the above-mentioned silica glass of the present invention.

Furthermore, the exposure apparatus (exposing device) of the presentinvention is an exposure apparatus which uses light in a wavelengthregion of 400 nm or shorter as exposure light and is characterized inthat it is provided with the optical member including theabove-mentioned silica glass of the present invention.

Moreover, the method for producing the silica glass in accordance withthe present invention is characterized in that it comprises the steps ofheating a silica glass ingot having an OH group concentration of atleast 1,000 ppm to a temperature of 1,200 to 1,350 K, maintaining theingot at that temperature for a predetermined period of time, and thencooling the ingot to a temperature of 1,000 K or lower at atemperature-lowering rate (cooling rate) of 50 K/hr or less to annealthe ingot, whereby making it possible to produce a silica glass having astructure determination temperature of 1,200 K or lower and an OH groupconcentration of at least 1,000 ppm.

The “structure determination temperature” herein used is a factorintroduced as a parameter which expresses structural stability of silicaglass and will be explained in detail below. The fluctuation in densityof silica glass at room temperature, namely, structural stability isdetermined by density of the silica glass in the state of melt at hightemperatures and density and structure of the silica glass when thedensity and the structure are frozen at around the glass transitionpoint in the process of cooling. That is, thermodynamic density andstructure corresponding to the temperature at which the density andstructure are frozen are also retained at room temperature. Thetemperature when the density and structure are frozen is defined to be“structure determination temperature” in the present invention.

The structure determination temperature can be obtained in the followingmanner. First, a plurality of silica glass test pieces are retained at aplurality of temperatures within the range of 1073-1700 K for a periodlonger than the structure relaxation time (a time required for thestructure of the silica glass being relaxed at that temperature) in theair in a tubular oven as shown in the accompanying FIG. 1, thereby toallow the structure of the respective test pieces to reach the structureat the retention temperature. As a result, each of the test pieces has astructure which is in the thermal equilibrium state at the retentiontemperature. In FIG. 1, 101 indicates a test piece, 102 indicates asilica glass tube, 103 indicates a heater, 104 indicates a thermocouple,105 indicates a beaker, and 106 indicates liquid nitrogen.

Then, the test pieces are introduced not into water, but into liquidnitrogen in 0.2 second to quench them. If they are introduced intowater, quenching is insufficient and structural relaxation occurs in theprocess of cooling, and the structure at the retention temperaturecannot be fixed. Moreover, it can be considered that adverse effect mayoccur due to the reaction between water and the silica glass. In thepresent invention, super-quenching can be attained by introducing thetest pieces into liquid nitrogen as compared with introduction intowater, and by this operation, it becomes possible to fix the structureof the test pieces to the structure at the retention time. In this way,for the first time, the structure determination temperature can beallowed to coincide with the retention temperature.

The thus obtained test pieces having various structure determinationtemperatures (equal to the retention temperatures here) are subjected tomeasurement of Raman scattering, and 606 cm⁻¹ line intensity is obtainedas a ratio to 800 cm⁻¹ line intensity. A graph is prepared withemploying as a variable the structure determination temperature for 606cm⁻¹ line intensity and this is used as a calibration curve. A structuredetermination temperature of a test piece of which the structuredetermination temperature is unknown can be inversely calculated fromthe measured 606 cm⁻¹ line intensity using the calibration curve. In thepresent invention, a temperature obtained in the above manner on asilica glass the structure determination temperature of which is unknownis employed as the structure determination temperature of the silicaglass.

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of an apparatus used formeasuring the structure determination temperature in accordance with thepresent invention;

FIG. 2 is a block diagram showing the basic structure of an example ofthe exposure apparatus of the present invention.

FIG. 3 is a conceptual view showing an integrating sphere typemeasurement apparatus for scattering light;

FIG. 4 is a conceptual view showing a goniophotometry type measurementapparatus for scattering light;

FIG. 5 is a conceptual view showing an ellipsoidal mirror typemeasurement apparatus for scattering light;

FIG. 6 is a graph showing the relationship between scattering lossamount and contrast;

FIG. 7 is a graph showing the relationship between wavelength andscattering loss;

FIG. 8 is a graph showing the relationship between structuredetermination temperature and scattering loss;

FIG. 9 is a graph showing the relationship between refractive index andscattering loss;

FIG. 10 is a schematic view showing an example of an apparatus forproducing a silica glass ingot in accordance with the present invention;

FIG. 11 is a bottom view showing an example of a burner for producing asilica glass ingot in accordance with the present invention;

FIG. 12 is a perspective view showing an example of an annealing furnacein accordance with the present invention; and

FIG. 13 is a graph showing the relationship between structuredetermination temperature and scattering loss.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the silica glass in accordance with the present invention will beexplained.

The silica glass in accordance with the present invention is used forphotolithography together with light in a wavelength region of 400 nm orshorter and is characterized in that it has a structure determinationtemperature of 1,200 K or lower and an OH group concentration of atleast 1,000 ppm and preferably within the range of 1,000 to 1300 ppm.

In this manner, when the structure determination temperature of 1,200 Kor lower and the OH group concentration of at least 1,000 ppm arespecified, a silica glass having a low scattering loss amount, such as ascattering loss amount of 0.2%/cm or less with respect to ArF excimerlaser, which cannot have been achieved conventionally, can be obtained,thereby sufficiently preventing the contrast from decreasing due toflare and ghost resulting from scattering light.

In general, optical energy impinging on an object generates a scatteringphenomenon. The scattering phenomenon can be roughly divided intoelastic scattering such as Rayleigh scattering and Brillouin scatteringand inelastic scattering such as Raman scattering. In particular, whenthe scattering intensity in an optical member is high, the scatteringlight therefrom becomes flare or ghost so as to decrease the contrast ofthe image, thereby causing the optical characteristic to deteriorate.

The optical scattering, however, has been considered to be sufficientlyless influential than the lowering of the resolution caused by change inthe form or refractive index of the optical member upon opticalabsorption and at a level which can be practically neglected. Actually,in optical instruments using light in the visible region, the main causefor transmission loss is optical absorption and, accordingly, when itsoptical absorption is set to a level not higher than a certain level, adesirable resolution is satisfied together with a favorable contrast inthe image.

The inventors, however, have found that, the optical scattering canbecome less negligible as the wavelength of the incident light has ashorter wavelength, so that, in particular, in the conventional opticalmembers such as projection lens used for photolithography, a vivid imagecannot be obtained due to flare and ghost resulting from the scatteringlight.

Though there has not been definitely elucidated mechanism by which thescattering loss with respect to ArF excimer laser is remarkablysuppressed when at least 1,000 ppm of OH group are introduced into asilica glass having a structure determination temperature of 1,200 K orless, i.e., a silica glass having nearly an ideal structure, theinventors consider as explained in the following. Meanwhile, thestructure determination temperature of the silica glass in accordancewith the present invention is quite lower than that of optical fibers,for example, which is typically about 1,450 K.

The silica glass having a high structure determination temperature isconsidered to be structurally unstable. Namely, the bond angle of≡Si—O—Si≡ in the network of silica glass has a certain distributionbecause of it being a glass and this distribution of the bond angleincludes structurally unstable bond angles. This distribution of thebond angle comprises bridged tetrahedrons made of oxygen atoms andsilicon atoms in the silicon glass and the presence of the unstable bondangles is considered to be caused by the presence of the tetrahedrons ina distorted state. Such a distorted bond portion is considered to bereadily cut by irradiation with ultraviolet light to produce the defectssuch as detrimental E′ center and NBOHC. On the other hand, the silicaglass having a low structure determination temperature is considered tohave few such distorted bond portions.

Also, the silica glass having an OH group iS concentration within theabove-mentioned range is structurally more stable than other kinds ofsilica glass and its structure determination temperature tends to becomelower.

The detailed reasons therefor are as follows. Namely, as mentionedabove, the ≡Si—O—Si≡ bond angle in the network of silica glass has acertain distribution because of it being a glass and it containsstructurally unstable distorted bond portions. However, when OH groupwithin the above-mentioned range is contained therein, there is no needto make bridges using unstable bond angle and, accordingly, thetetrahedron can approximate its most stable structure.

Therefore, the silica glass containing OH group within theabove-mentioned range is structurally more stable than other kinds ofsilica glass and its structure determination temperature tends to becomelower.

Accordingly, in the silica glass in accordance with the presentinvention in which the OH group concentration is at least 1,000 ppm andthe structure determination temperature is 1,200 K or lower, due totheir synergistic effect, the scattering loss amount of 0.2%/cm or lessis attained with respect to ArF excimer laser.

Preferably, the silica glass in accordance with the present inventionhas a fluorine content of at least 300 ppm. This is because, under thesame annealing condition, the structure determination temperature tendsto become lower when the fluorine content is at least 300 ppm.

Further, the total amount of optical scattering and optical absorption,namely, the transmission loss amount, influences the light amount on thereticle and wafer, thereby influencing the decrease in throughput due tothe decrease in illumination intensity or the like. In particular, inthe photolithography optical system, since its resolution is maximizedto the limit, the number of lens sheets for correcting various kinds ofwave front aberrations is large and its optical length is long.Accordingly, even a minute transmission loss amount (scattering lossamount plus absorption loss amount) may become influential. For example,in the optical path length of 1 m, even when the transmission lossamount is only 0.2%/cm, the total transmission loss amount becomes about18%.

Therefore, in the silica glass in accordance with the present invention,the internal absorptivity in the silica glass having a thickness of 10mm with respect to ArF excimer laser is preferably 0.2%/cm or less. Thiskind of optical absorption is a cause for decreasing the resolution aswill be explained in the following. Namely, optical absorption is aphenomenon resulting from electronic transition caused by photon energyimpinging on an optical member. When optical absorption occurs in theoptical member, its energy is mainly converted into thermal energy,thereby inflating the optical member or changing its refractive index orsurface condition. As a result, high resolution cannot be obtained.Further, the optical absorption is accompanied by change in electroniccondition and, during the period by which it is relaxed, light having alonger wavelength than that of the incident light is released asfluorescence. When the fluorescence has a wavelength similar to that ofexposure light and its intensity is high, the contrast of the image isremarkably decreased. Accordingly, in order to obtain a fine and vividimage with a favorable contrast, it is preferable to specify theabsorption loss amount together with the scattering loss amount.

Also, as factors for deteriorating the ultraviolet light resistance ofsilica glass, there have been known, for example, ≡Si—Si≡, ≡Si—O—O—Si≡,and dissolved oxygen molecules. These precursors are readily convertedinto structural defects such as E′ center and NBOHC upon exposure toultraviolet light such as excimer laser, thereby causing thetransmittance to decrease. In the silica glass in accordance with thepresent invention, it is preferable that there are no incompletestructures caused by such a deviation from the stoichiometric ratio. Forexample, when OH group within the above-mentioned range is containedtherein, the silica glass tends to contain substantially nooxygen-shortage type defect absorption bands (7.6- and 5.0-eV absorptionbands). Also, when the silica glass in accordance with the presentinvention containing at least 5×10¹⁶ molecules/cm³ of hydrogen moleculesis irradiated with 1×10⁶ pulses of ArF excimer laser at a one-pulseenergy density of 100 mJ/cm², substantially no oxygen-excess type defectabsorption band (4.8-eV absorption band) is generated. Due to theabsence of these defects, according to measurement of transmittanceeffected by vacuum ultraviolet, ultraviolet, visible, and infraredspectrophotometers, high transmittance ratios of at least 99.9% in termsof internal transmittance (for silica glass having a thickness of 10 mm)for the light of the wavelength of g-line (436 nm) to i-line (365 nm)and KrF excimer laser beam (248 nm) and at least 99.6% in terms ofinternal transmittance (for silica glass having a thickness of 10 mm)for the light of the wavelength of ArF excimer laser beam (193 nm).Also, after being irradiated with 1×10⁶ pulses of KrF excimer laser atan average one-pulse energy density of 400 mJ/cm², the internaltransmittance of the above-mentioned silica glass having a thickness of10 mm exceeds 99.5% with respect to the light having a wavelength of 248nm. On the other hand, after being irradiated with 1×10⁶ pulses of ArFexcimer laser at an average one-pulse energy density of 100 mJ/cm², theinternal transmittance of the above-mentioned silica glass having athickness of 10 mm exceeds 99.5% with respect to the light having awavelength of 193 nm.

Also, it is desirable for the structure determination temperaturedistribution in the silica glass of the present invention to have acenter symmetry within the member since this will render a centersymmetry to its scattering loss characteristic (scattering intensity).In this case, it becomes easy to specify, at the time of adjusting alens, lens parts which may cause flare or ghost, thereby facilitatingthe optical adjustment. Further, the contrast can be prevented fromfluctuating on the image-forming surface. Moreover, the silica glass ofthe present invention preferably has a birefringence amount of 2 nm/cmor less and centrally symmetrical polarization and birefringencecharacteristics.

In the silica glass of the present invention, the chlorine concentrationis preferably 50 ppm or less and, in particular, 10 ppm or less. This isbecause, when the chlorine concentration exceeds 50 ppm, it tends tobecome difficult for the OH group concentration in the silica glass tobe maintained at 1,000 ppm or higher.

Also, the silica glass preferably has a high quality such that itsconcentration of each of metallic impurities (Mg, Ca, Ti, Cr, Fe, Ni,Cu, Zn, Co, Mn, Na and K) is 50 ppb or less and more preferably 20 ppbor less. In this case, the above-mentioned structural defects decreaseto form a structure approximating the ideal structure and also thechange in refractive index, change in surface, and deterioration oftransmittance caused by the metallic impurities become lower such thatthe ultraviolet light resistance tends to improve.

In the following, the optical member and exposure apparatus of thepresent invention will be explained. The optical member of the presentinvention includes the above-mentioned silica glass of the presentinvention in which the structure determination temperature is 1,200 K orless and the OH group concentration is at least 1,000 ppm. Such anoptical member has no particular limitation, as long as it includes theabove-mentioned silica glass, and may be such an optical member as lensor prism which is used together with light in a wavelength region of 400nm or shorter. The optical member of the present invention includesblank. Further, the method for processing the above-mentioned silicaglass of the present invention into the optical member of the presentinvention is not restricted in particular, while normal cutting methodor abrasion method, for example, may be appropriately used.

Since the optical member of the present invention includes a silicaglass which, as mentioned above, exhibits a very small scattering lossamount with respect to light of a short wavelength such as ArF excimerlaser beam, as compared with the conventional optical members, thescattering light is more effectively prevented from generating and itexhibits a higher resolution. Accordingly, the silica glass of thepresent invention is suitably applied to an optical member such as alens in a projection optical system of steppers which requires such ahigh resolution as 0.25 μm or less. The silica glass of the presentinvention is useful not only for lenses in the projection system of thestepper but also for lenses in illumination optical systems, forexample.

An exposure apparatus of the present invention will be described next.The exposure apparatus of the present invention is provided with theoptical member comprising the silica glass of the present invention anduses light in the wavelength region of 400 nm or shorter as exposurelight, and has no limitation except that it contains the silica glass asa lens of illumination optical system, projection optical system, or thelike, and is provided with a light source for emitting light in thewavelength region of 400 nm or shorter.

The present invention is preferably applied to the projection exposureapparatus, such as a so-called stepper, for projecting an image ofpatterns of reticle onto a wafer coated with a photoresist.

FIG. 2 shows a basic structure of the exposure apparatus according tothe present invention. As shown in FIG. 2, an exposure apparatus of thepresent invention comprises at least a wafer stage 301 allowing aphotosensitive substrate W to be held on a main surface 301 a thereof,an illumination optical system 302 for emitting vacuum ultraviolet lightof a predetermined wavelength as exposure light and transferring apredetermined pattern of a mask (reticle R) onto the substrate W, alight source 303 for supplying the exposure light to the illuminationoptical system 302, a projection optical system (preferably acatadioptric one) 304 provided between a first surface P1 (object plane)on which the mask R is disposed and a second surface P2 (image plane) towhich a surface of the substrate W is corresponded, for projecting animage of the pattern of the mask R onto the substrate W. Theillumination optical system 302 includes an alignment optical system 305for adjusting a relative positions between the mask R and the wafer W,and the mask R is disposed on a reticle stage 306 which is movable inparallel with respect to the main surface of the wafer stage 301. Areticle exchange system 307 conveys and changes a reticle (mask R) to beset on the reticle stage 306. The reticle exchange system 307 includes astage driver for moving the reticle stage 306 in parallel with respectto the main surface 301 a of the wafer stage 301. The projection opticalsystem 304 has a space permitting an aperture stop 308 to be settherein. The sensitive substrate W comprises a wafer 309 such as asilicon wafer or a glass plate, etc., and a photosensitive material 310such as a photoresist or the like coating a surface of the wafer 309.The wafer stage 301 is moved in parallel with respect to a object planeP1 by a stage control system 311. Further, since a main control section312 such as a computer system controls the light source 303, the reticleexchange system 307, the stage control system 311 or the like, theexposure apparatus can perform a harmonious action as a whole.

The exposure apparatus of the present invention comprises an opticalmember which comprises the silica glass of the present invention, forexample an optical lens consisting of the above-mentioned silica glass.More specifically, the exposure apparatus of the present invention shownin FIG. 2 can include the optical lens of the present invention as anoptical lens 313 in the illumination optical system 302 and/or anoptical lens 314 in the projection optical system 304.

Since the exposure apparatus of the present invention is provided withthe optical member made of a silica glass which, as mentioned above,exhibits a very small scattering loss amount with respect to light of ashort wavelength such as ArF excimer laser beam, as compared with theconventional optical members, the contrast of the image is moresufficiently prevented from lowering due to flare or ghost and itattains a higher resolution.

The techniques relating to an exposure apparatus of the presentinvention are described, for example, in U.S. patent application Ser.No. 255,927, No. 260,398, No. 299,305, U.S. Pat. No. 4,497,015, No.4,666,273, No. 5,194,893, No. 5,253,110, No. 5,333,035, No. 5,365,051,No. 5,379,091, or the like. The reference of U.S. patent applicationSer. No. 255,927 teaches an illumination optical system (using a lasersource) applied to a scan type exposure apparatus. The reference of U.S.patent application Ser. No. 260,398 teaches an illumination opticalsystem (using a lamp source) applied to a scan type exposure apparatus.The reference of U.S. patent application Ser. No. 299,305 teaches analignment optical system applied to a scan type exposure apparatus. Thereference of U.S. Pat. No. 4,497,015 teaches an illumination opticalsystem (using a lamp source) applied to a scan type exposure apparatus.The reference of U.S. Pat. No. 4,666,273 teaches a step-and repeat typeexposure apparatus capable of using the catadioptric projection opticalsystem of the present invention. The reference of U.S. Pat. No.5,194,893 teaches an illumination optical system, an illuminationregion, mask-side and reticle-side interferometers, a focusing opticalsystem, alignment optical system, or the like. The reference of U.S.Pat. No. 5,253,110 teaches an illumination optical system (using a lasersource) applied to a step-and-repeat type exposure apparatus. The '110reference can be applied to a scan type exposure apparatus. Thereference of U.S. Pat. No. 5,333,035 teaches an application of anillumination optical system applied to an exposure apparatus. Thereference of U.S. Pat. No. 5,365,051 teaches a auto-focusing systemapplied to an exposure apparatus. The reference of U.S. Pat. No.5,379,091 teaches an illumination optical system (using a laser source)applied to a scan type exposure apparatus. These documents are herebyincorporated by reference.

Having conducted optical simulation and experiments for evaluatingimage-forming properties in cases where the above-mentioned opticalmember is used, the inventors have found it possible to provide anexposure apparatus (photolithography apparatus) which is substantiallyprevented from being influenced by flare or ghost and there is noproblem concerning the decrease of light amount in terms of itsproperty.

Based on this finding, in an optical system constructed by using theoptical member of the present invention, a fine and vivid exposure andtransfer pattern with a line width of 0.25 μm or less has been obtained.

Thus, the inventors have diligently studied characteristics of theoptical member by which a fine and vivid exposure and transfer patterncan be obtained in photolithography techniques. As a result, theinventors have found that, among optical properties of a projectionlens, the transmission loss amount is quite influential to theimage-forming property in cases where the uniformity in refractive index(Δn), lens surface accuracy, and optical thin-film characteristic are atsubstantially the level and, more importantly, that the opticalproperties of the projection lens cannot be correctly expected unlessits transmission loss is separated into optical absorption and opticalscattering so as to be precisely evaluated. This is because the opticalabsorption and the optical scattering generate phenomenons differentfrom each other, namely, the former contributes to the deterioration ofthe image-forming property resulting from the heating within the lens,whereas the latter contributes to the deterioration of the contrastresulting from flare or ghost.

Here, explanation will be provided in detail for the optical scatteringin the optical member.

An optical single crystal such as single crystal fluorite (CaF₂) isregarded as a perfect crystal. Namely, it is assumed that the wholeatoms and ions in the crystal are regularly arranged with a distance ofabout 5 Å therebetween and that the crystal has a uniform density. Also,in view of Huygens-Fresnel's principle concerning the propagation oflight, even when the wave front of light collides with a molecule (i.e.,scattering factor) to generate numerous secondary spherical waves,except for the scattering light in the direction where light travelsstraight ahead, these waves interfere with each other so as to canceleach other. Accordingly, the scattering loss of the optical singlecrystal becomes quite smaller than that of liquid as well as that ofglass or plastic which is in a non-equilibrium state and, whenstructural defects, fine particles, and the like do not practicallyexist therewithin, its scattering loss amount is considered to benegligible.

However, since a melt material is rapidly cooled when manufacturing aglass, the arrangement of atoms in the melt glass may be maintained to acertain degree in the cooled glass. Accordingly, while the glass is asolid in terms of a macroscopic property, it has a structure of liquidmicroscopically. Therefore, like liquid, it is considered that themolecular distribution of the glass does not have a regularity such asthat of a crystal and has a statistical thermodynamic fluctuation due toits thermal motion, thereby generating optical scattering. Such opticalscattering is known as Rayleigh scattering.

In Rayleigh scattering, the scattering intensity is inverselyproportional to the fourth power of wavelength λ. Accordingly, inoptical instruments used in a short wavelength region, Rayleighscattering of its optical member is influential to the opticalproperties. In particular, optical instruments such as projection lensesused for photolithography where a superfine resolution is required,flare and ghost caused by transmission loss and scattering light becomeproblematic.

Scattering loss amount of a glass can be calculated by the followingequation: $\begin{matrix}{s = {\frac{8\pi^{3}}{3\lambda^{4}}n^{8}p^{2}{kTs\beta T}}} & (3)\end{matrix}$

wherein

s: scattering loss coefficient (/cm)

p: Pockels coefficient 0.27

Ts: structure determination temperature (K)

βT: isothermal compressibility 7E-12 (cm/dyn)

ρ: density 2.201 (g/cm³)

λ: wavelength (cm)

k: Boltzmann constant 1.38E-16 (erg/K)

n: refractive index

For example, when calculation is made with respect to a silica glass,for given physical property values of wavelength λ=193.4 nm, refractiveindex n=1.5603, and structure determination temperature Ts=1,273 K; thescattering loss coefficient is calculated as s=0.001861/cm, namely, thescattering loss amount is calculated as 0.1861%/cm. In this manner, theinventors have found that a larger amount is expected than the actuallymeasured transmission loss amount and that the main cause for thetransmission loss at 193.4 nm is more attributable to scattering lossthan optical absorption.

Here, in order to correct the Brillouin scattering portion and tocalculate the Rayleigh scattering coefficient, the term of βT iscorrected as follows:

βT→[βT−(ρv∞ ²)⁻¹]=5.7 E-12  (4)

wherein

v∞: high-frequency sound velocity 5.92 (cm/s)

As a result of this calculation, the scattering loss amount becomes0.1516%/cm.

In view of the foregoing, the theoretically calculated value of thescattering loss is defined as the Rayleigh scattering loss plus theBrillouin scattering loss. Here, the Brillouin scattering loss can becalculated when equation (3) is used while using (v∞²)⁻¹ shown inequation (4) in place of βT and setting Ts at room temperature (298 K).Brillouin scattering is theoretically estimated as about {fraction(1/20)} with respect to Rayleigh scattering.

However, thus obtained scattering loss amount may have been estimatedlower than its actual value since other scattering factors and inelasticscattering, for example, are not considered. Also, since the valuesindicated here are calculated from theoretical equations and there maybe a problem concerning reliability of physical property values, theyshould be regarded as nothing other than estimated values.

Therefore, in practice, it is necessary for the scattering loss amountto be measured.

Here, an apparatus for measuring the scattering loss amount will beexplained in detail.

Examples of the measurement apparatus include i) integrating sphere type(FIG. 3) which uses an integrating sphere to measure the totalscattering amount; ii) goniophotometry type (FIG. 4) used for measuringangular distribution; and iii) ellipsoidal mirror type (FIG. 5) whichuses an ellipsoidal mirror.

Among the above-mentioned types, substantially the common light sourceand optical system are used. With respect to the visible light region,there is used an actual measurement technique in which He-Ne laser(632.8 nm), Ar⁺ ion laser (e.g., 488 nm) and the like are used as thelight source. With respect to the actual wavelength of ArF excimer laser(193.4 nm), there is used an actual measurement technique in which D2lamp, ArF excimer laser and the like are used as the light source; oranother technique in which Hg lamp emission line is used so as tointerpolate the scattering loss amount at 193.4 nm according to acalculation equation.

Preferably, a sample has a cylindrical or prismatic form in whichlight-input and light-output surfaces are parallel planes, while theother surfaces preferably have a surface roughness of 5 Å or less interms of RMS and a high surface cleanliness. These characteristics areused in order to eliminate the influence of the surface scattering andsurface absorption.

The optical scattering and optical absorption in the present inventionrefer to internal scattering and internal absorption of an opticalmember, respectively. In the following, the detection means in each typeof the measurement apparatus will be explained.

In the type shown in FIG. 3 in which an integrating sphere is used, asample (object to be tested) is held at an optical path portion withinthe integrating sphere. In this case, it is preferable that the lengthof the sample is slightly longer than that of the optical path lengthwithin the integrating sphere. This feature is used in order to preventsurface-scattered light from entering the integrating sphere.

Also, in order to block a measurement system from surface-reflected andsurface-scattered light components, the parallel plane portion isprovided with a wedge of a few minutes or the system is tilted by a fewdegrees with respect to the optical axis. Further, a signal intensityobtained without the sample is used for zero-point calibration while anND filter or the like whose transmittance is accurately secured is usedfor determining a calibration curve. As an optical detection device, aphotodiode or photomultiplier, for example, which is highly sensitiveand stable at each measurement wavelength is used.

FIG. 4 shows an apparatus in which goniophotometry technique is used formeasuring, in principle, angular dependency of scattering light. Inorder to use thus configured apparatus to measure the absolute value ofthe scattering light, such a value in the visible light region iscalculated on the basis of its relative value with respect to a materialsuch as benzene whose scattering loss coefficient is known. In theultraviolet region, rare gas or the like which is hard to be influencedby optical absorption is preferably used.

For example, based on a relative scattering intensity comparison of θ90degrees with respect to the optical axis (R90 ratio: intensity of90-degree direction with respect to optical axis), the whole scatteringamount can be estimated by:

16π/3×R90

In this case, it is assumed that the angular dependency of thescattering is of complete Rayleigh scattering.

A light-input portion of an optical fiber is set in a θ90-degreedirection with respect to the optical axis in order to transfer thescattering light to the detection means, while a spectrometer using aphotodiode array is employed as the detection means, thereby enablingeasy measurement of the R90 relative value. Also, the spectrum of thescattering light can be confirmed.

The ellipsoidal mirror type apparatus shown in FIG. 5 is mainly used formeasuring the surface scattering. While this apparatus is excellent inmeasuring relative intensity in the measurement of scattering as well,it is disadvantageous, for example, in that a complicated correctionequation is required for calculating the absolute value.

In view of the foregoing, using the actually measured scattering valuesattained by the integrating sphere type and goniophotometry typeapparatuses, the inventors have studied the influences of the scatteringloss upon optical properties of a photolithography apparatus such as itsresolution and contrast. Also, based on the results thereof, opticalsimulation and experiments for evaluating image-forming property havebeen effected.

FIG. 6 shows the relationship between the total scattering loss amountand contrast in an optical system in the photolithography apparatusobtained by the experiments for evaluating image-forming property. Asshown in this chart, a very good correlation has been obtained betweenthem.

Here, the standard value of the scattering loss amount, 0.2%/cm, is avalue calculated from the following equation: $\begin{matrix}{\frac{S0}{L} \simeq {0.2\left( {\%/\text{cm}} \right)}} & (5)\end{matrix}$

wherein

S0: maximum value (%) of total scattering loss allowed for obtaining arequired contrast

L: total optical path length of optical system (cm)

total scattering loss=(total scattering loss intensity)/(incident lightintensity)×100 (%)

Namely, the image-forming evaluation experiments have confirmed that theimage-forming property of a stepper is remarkably influenced by not onlythe absorption loss but scattering loss and that, when the scatteringloss amount is 0.2%/cm or less, flare and ghost have substantially noinfluence and the decrease in light amount is at a level which is notproblematic in terms of the property without influencing theimage-forming property.

Further, when the scattering loss amount is 0.2%/cm or less with respectto light of 193.4 nm, since the scattering loss amount is inverselyproportional to wavelength λ⁴ while being proportional to refractiveindex n⁸ the scattering loss amount becomes smaller as the wavelength islonger for light in a wavelength region longer than 193.4 nm, therebysatisfying the standard required for the present invention. This facthas been confirmed by equation (3) as well as by the results of theexperiments.

By contrast, from the scattering loss amount in a visible region, forexample, at wavelengths of He-Ne laser (632.8 nm) and Ar⁺ ion laser(e.g., 488 nm), the scattering loss amount at 193.4 nm can be calculatedby using the inversely proportional rule with respect to wavelength λ⁴and the proportional rule with respect to refractive index n⁸, therebyjudging whether the standard in accordance with the present invention,i.e., the scattering loss amount of 0.2%/cm or less, is satisfied ornot.

FIG. 7 shows the results obtained when the actually measured values ofvarious kinds of silica glass used for photolithography are comparedwith theoretically calculated values of scattering loss amountcalculated by using equation (3) in which structure determinationtemperature Ts is set at 1,273 K. As shown in this chart, the actuallymeasured values are higher than the theoretically calculated values andexhibit a large fluctuation. It has been found that, at 193.4 nm, due tosuch a fluctuation, the conventionally used silica glasses forphotolithography exceed the standard for the scattering loss amount inthe present invention which is 0.2%/cm or less. By contrast, as can beseen from Examples which will be described later, the silica glass ofthe present invention can attain a scattering loss amount of 0.2%/cm orless even with respect to light of 193.4 nm.

Also, the inventors have confirmed the relationship between structuredetermination temperature Ts and the scattering loss. FIG. 8 shows theresults thereof. Here, the actually measured values are also slightlyhigher than the theoretical values. This phenomenon is assumed to be theresults of optical scattering (e.g., influence of particulate orcolloidal scattering factors such as optical glass and influence ofinelastic scattering) and lack of reliability in physical propertiesused for theoretical calculation.

Further, the inventors have confirmed the relationship between thescattering loss and the change in refractive index caused by change inOH group and F concentration in the silica glass and HIP processing.FIG. 9 shows the results thereof. As shown in this chart, the actuallymeasured values are higher than the theoretically calculated values.Also, the scattering loss amount has been found to be dependent onrefractive index. Further, it has been discovered that, in order tosatisfy the standard for the scattering loss in accordance with thepresent invention which is 0.2%/cm or less, the refractive index ispreferably less than 1.56 with respect to light of 193.4 nm.

Next, the method for producing the silica glass of the present inventionwill be explained.

In the method for producing the silica glass of the present invention, asilica glass ingot having an OH group concentration of 1,000 ppm or moreis heated to a temperature of 1200-1350 K and the ingot is retained atthis temperature for a given period of time. When the retentiontemperature exceeds 1350 K, the surface of the silica glass is degraded,and it would become necessary to spend very long period of time forlowering the structure determination temperature of the silica glass to1200 K or lower. When the retention temperature is lower than 1200 K,the structure determination temperature cannot be lowered to 1200 K orlower in a given period, and, furthermore, annealing is insufficient andstrain cannot be removed. The retention time is preferably a period oflonger than the structure relaxation time at the retention temperature,especially preferably 1-24 hours. For example, in the case of the silicaglass having a structure determination temperature of 1300 K or higherand containing OH group in an amount of about 1000 ppm, the structurerelaxation time at 1273 K is 280 seconds. The heating rate(temperature-rising rate) does not affect the properties of theresulting silica glass, but is preferably less than about 150 K/hr.

Then, the above silica glass ingot is cooled to a temperature (annealingcompletion temperature (a.c.t.) 25) of 1000 K or lower, preferably 873 Kor lower, more preferably 473 K or lower, at a cooling rate (annealingrate or temperature-lowering rate for annealing) of 50 k/hr or less,preferably 20 K/hr or less, thereby to anneal the ingot. When theannealing completion temperature is higher than 1000 K or the annealingrate (temperature-lowering rate) is higher than 50 k/hr, the structuredetermination temperature cannot be lowered to 1200 K or lower and,besides, strain cannot be sufficiently removed.

After the ingot reaches the above annealing completion temperature,usually it is air-cooled or spontaneously cooled to room temperature,though this is not essential. The atmosphere of the above annealing stepis unlimited and may be air. The pressure is also unlimited and may beatmospheric pressure.

Further, the method of the present invention preferably additionallyincludes, prior to the annealing step, a step of hydrolyzing a siliconcompound such as SiCl₄, SiHCl₃, SiF₄,or the like in a flame (preferablyoxy-hydrogen flame) to obtain fine glass particles (glass soot) anddepositing and fusing the fine glass particles to obtain the silicaglass ingot having an OH group concentration of 1,000 ppm or more.

Moreover, preferably, the method of the present invention furthercomprises a step of descending the temperature of the above-mentionedsilica glass ingot from a temperature of at least 1,373 K to atemperature of 1073 K or lower, preferably to a temperature of 773 K orlower, more preferably to room temperature, at a rate of 50 K/hr orless, preferably 20 K/hr or less, and more preferably 10 K/hr, therebyto pre-anneal the silica glass ingot. When the silica glass ingot ispre-annealed in this manner, the structure determination temperature ofthe silica glass tends to become lower.

As mentioned above, the silica glass ingot of the present invention ispreferably prepared by the above-mentioned direct method, namely,oxy-hydrogen flame hydrolysis. That is, ≡Si—Si≡ bond, ≡Si—O—O—Si≡ bondand the like are known as the precursors which cause formation ofstructural defects when synthetic silica glass is irradiated withultraviolet light, and synthetic glasses obtained by so-called sootmethods (VAD method, OVD method) or plasma method have these precursors.On the other hand, synthetic silica glasses produced by the directmethod have no incomplete structures of oxygen-shortage or -excessivetype formed by deviation from the stoichiometric ratio. Furthermore,high purity of low metallic impurities can generally be attained in thesynthetic silica glasses produced by the direct method. Moreover, sincethe silica glasses synthesized by the direct method generally containmore than several hundred ppm of OH group, they are structurally morestable as compared with silica glasses containing no OH group.

The silica glass synthesized by the so-called direct method whichcomprises hydrolyzing silicon chloride with oxy-hydrogen flame anddepositing the resulting silica fine glass particles on a target andmelting it to form a silica glass ingot has a structure determinationtemperature of 1300 K or higher at the state just after synthesis.

In order to obtain a silica glass ingot having an OH group concentrationof 1,000 ppm or more by the direct method, it is preferred that thevolume ratio of oxygen gas to hydrogen gas (O₂/H₂) in the flame is atleast 0.4, more preferably 0.42-0.5. When this ratio (oxygen hydrogengas ratio) is less than 0.4, it tends to occur that the resulting silicaglass ingot does not contain 1,000 ppm or more of OH group.

Furthermore, in the method of the present invention, the effect of theabove-mentioned annealing is attained more effectively and uniformly bycutting the silica glass ingot to make blanks having a given size,preferably 200-400 mm in diameter and 40-150 mm in thickness, and then,annealing them.

EXAMPLES 1-14 AND COMPARATIVE EXAMPLES 1-10

A silica glass ingot was produced using the apparatus for producingsilica glass as shown in FIG. 10. That is, high purity silicontetrachloride A (starting material) (Examples 1-11 and ComparativeExamples 1-8), or silicon tetrachloride A and silicon tetrafluoride B(starting material) (Examples 12-14 and Comparative Examples 9-10),which are fed from silicon compound bomb 401, was mixed with a carriergas fed from oxygen gas bomb 403 in baking system 402, and the mixturewas fed to silica glass burner 406 together with hydrogen gas fed fromhydrogen bomb 404 and oxygen gas fed from oxygen gas bomb 405. Oxygengas and hydrogen gas in flow rates shown in Table 1 were mixed and burntin the burner 406 and the starting material gas in the flow rate shownin Table 1 was diluted with a carrier gas (oxygen gas) and ejected fromthe central part to obtain silica fine glass particles (SiO₂ fineparticles). The silica fine glass particles were deposited and molten ontarget 408 surrounded by refractory 407 and then cooled to roomtemperature at a temperature-lowering rate (cooling rate atpre-annealing) shown in Table 2 thereby to obtain silica glass ingot 409(500 mm long) having the composition shown in Table 1. In this case, theupper face (synthesis face) was covered with flame and the target 408was lowered at a constant speed with rotating and rocking at a constantperiod. The structure determination temperature of the silica glass atthis stage was 1400 K. The reference numeral 410 in FIG. 10 indicates amass flow controller and R in Table 1 indicates an oxygen hydrogen ratio(O₂/H₂).

The burner 406 has quintuple tube structure as shown in FIG. 11, and 501indicates an ejection port for starting material and carrier gas, 502indicates an ejection port for inner side oxygen gas (OI), 503 indicatesan ejection port for inner side hydrogen gas (HI), 504 indicates anejection port for outer side oxygen gas (OO), and 505 indicates anejection port for outside hydrogen gas (HO). The size (mm) of theejection port is as follows.

Inner diameter Outer diameter Burner A 501 6.0 9.0 502 12.0 15.0 50317.0 20.0 504 3.5 6.0 505 59.0 63.0 Burner B 501 3.5 6.5 502 9.5 12.5503 14.5 17.5 504 3.5 6.0 505 59.0 63.0 Burner C 501 2.0 5.0 502 8.511.5 503 14.5 17.5 504 3.5 6.0 505 59.0 63.0

Then, a test piece to be irradiated with ArF excimer laser beam (60 mmin diameter and 10 mm in thickness, the opposite two sides beingsubjected to optical abrasion) was prepared from each of the resultingingots. The test piece was placed in an annealing furnace made of aninsulating firebrick as shown in FIG. 12 and heated to retentiontemperature from room temperature at a heating rate shown in Table 2.After lapse of the retention time, it was cooled to the annealingcompletion temperature from the retention temperature at an annealingrate (temperature-lowering rate) shown in Table 2 and, thereafter,spontaneously cooled to room temperature. The cooling rate after a.c.t.shown in Table 2 is a cooling rate one hour after starting of thespontaneous cooling. Moreover, in FIG. 12, 601 indicates a test piece,602 indicates an annealing furnace, 603 indicates a stand comprising asilica glass and legs made of a firebrick, and 604 indicates a rod-likeSiC heating element.

TABLE 1 Inner Starting Material Diameter Gas Inner Side Outer SideGrowing Diame- of F Con- Flow Flow Oxygen · Hydrogen Oxygen · HydrogenRate ter Tube for Example/ H₂ cent- Rate Velocity Gas Gas of of StartingComparative Concentration ration [g/ [g/min/ HI OI HO OO Ingot IngotMaterial Example [mol./cm³] [ppm] Kind min] cm²] [slm] [slm] R [slm][slm] R [mm/hr] [mm] [mmφ] Ex. 1-6 About 1.1 × 10¹⁷ 0 A 30 330 70 30.80.44 200 78 0.44 0.6 200 6.0 Comp. Ex. 1-3 Ex. 7 <1 × 10¹⁶ 0 A 30 330 7035 0.5 200 100 0.5 0.6 200 6.0 Ex. 8 About 1.3 × 10¹⁷ 0 A 30 330 70 280.40 200 78 0.44 0.6 200 6.0 Ex. 9-11 About 1.5 × 10¹⁷ 0 A 30 330 7029.4 0.42 200 78 0.44 0.6 200 6.0 Comp. Ex. 4 Comp. Ex. 5-6 About 2.1 ×10¹⁸ 0 A 30 330 150 45 0.3 360 158 0.44 4.0 250 3.5 Comp. Ex. 7-8 About2.1 × 10¹⁸ 0 A 30 330 150 45 0.3 360 158 0.44 8.0 250 2.0 Ex. 12-13 <1 ×10¹⁶ 350 A 20 330 70 35 0.5 200 100 0.5 0.6 200 6.0 Comp. Ex. 9 B 6.1Comp. Ex. 10 About 2.1 × 10¹⁸ 350 A 20 330 150 45 0.3 360 158 0.44 4.0250 3.5 B 6.1 Ex. 14 About 2.1 × 10¹⁸ 800 A 10 330 150 45 0.3 200 780.44 0.6 200 6.0 B 12.2

TABLE 2 Annealing Pre- Annealing Cooling Annealing Heat- Reten- Reten-Anneal- Completion Rate Example/ Cooling ing tion tion ing TemperatureAfter Comparative Rate Rate Temp. Time Rate (a.c.t) a.c.t. Example[K/hr] [K/hr] [K] [hr] [K/hr] [K] [K/hr] Ex. 1 125 100 1223 10 1.0 77380 Ex. 2 125 100 1273 10 1.0 773 80 Ex. 3 125 100 1273 5 1.0 773 80 Ex.4 125 100 1273 10 5 773 80 Ex. 5 125 100 1273 10 10 773 80 Ex. 6 125 1001273 10 10 993 80 Comp. Ex. 1 125 100 1273 10 10 1173 80 Comp. Ex. 2 125100 1273 10 100 773 80 Comp. Ex. 3 125 100 1373 10 10 773 80 Ex. 7 50100 1273 10 7 773 80 Ex. 8 50 100 1273 10 10 773 80 Ex. 9 50 100 1273 1020 773 80 Comp. Ex. 4 50 100 1273 10 100 773 80 Ex. 10 125 100 1223 101.0 773 80 Comp. Ex. 5 125 100 1223 10 1.0 773 80 Ex. 11 125 100 1273 1010 773 80 Comp. Ex. 6 125 100 1273 10 10 773 80 Comp. Ex. 7 125 100 127310 100 773 80 Comp. Ex. 8 50 100 1373 10 10 773 80 Ex. 12 50 100 1223 101.0 773 80 Ex. 13 125 100 1223 10 10 773 80 Comp. Ex. 9 125 100 1373 10100 773 80 Comp. Ex. 10 125 100 1273 10 10 773 80 Ex. 14 125 100 1223 1010 773 80

TABLE 3 OH Group F Scatter- Example/ Concent- Concent- H₂ ingComparative Ts ration ration Concentration Loss Example [K] [ppm] [ppm][mol./cm³] [%/cm] Ex. 1 1023 1200 0 1 × 10¹⁷ 0.10 Ex. 2 1073 1200 0 1 ×10¹⁷ 0.12 Ex. 3 1123 1200 0 1 × 10¹⁷ 0.15 Ex. 4 1150 1200 0 1 × 10¹⁷0.17 Ex. 5 1173 1200 0 1 × 10¹⁷ 0.18 Ex. 6 1198 1200 0 1 × 10¹⁷ 0.19Comp. Ex. 1 1223 1200 0 1 × 10¹⁷ 0.23 Comp. Ex. 2 1273 1200 0 1 × 10¹⁷0.23 Comp. Ex. 3 1223 1200 0 <1 × 10¹⁶ 0.24 Ex. 7 1223 1250 0 <1 × 10¹⁶0.1 Ex. 8 1150 1010 0 1.2 × 10¹⁷ 0.15 Ex. 9 1193 1050 0 1.4 × 10¹⁷ 0.18Comp. Ex. 4 1223 1050 0 1.4 × 10¹⁷ 0.23 Ex. 10 1023 1050 0 1.4 × 10¹⁷0.11 Comp. Ex. 5 1023 900 0 2 × 10¹⁸ 0.23 Ex. 11 1173 1050 0 1.4 × 10¹⁷0.16 Comp. Ex. 6 1173 900 0 2 × 10¹⁸ 0.22 Comp. Ex. 7 1273 780 0 2 ×10¹⁸ 0.35 Comp. Ex. 8 1223 780 0 1 × 10¹⁷ 0.3 Ex. 12 1023 1200 350 <1 ×10¹⁶ 0.07 Ex. 13 1173 1200 350 <1 × 10¹⁶ 0.2 Comp. Ex. 9 1223 1200 350<1 × 10¹⁶ 0.4 Comp. Ex. 10 1173 900 350 2 × 10¹⁸ 0.21 Ex. 14 1123 1200800 2 × 10¹⁸ 0.10

Structure determination temperature (Ts), OH group concentration, Fconcentration and hydrogen molecule concentration of these test pieceswere measured. The results are shown in Table 3. The structuredetermination temperature was obtained by inversely calculating frommeasured 606 cm⁻¹ line intensity value based on the previously preparedcalibration curve. The hydrogen molecule concentration was measured by alaser Raman photometer. That is, among the Raman scattered lightsperpendicular to the sample which occurred when the sample wasirradiated with Ar+ laser beam (output 800 mW), intensity of 800 cm⁻¹and 4135 cm⁻¹ was measured and the ratio of the intensities wasdetermined. The OH group concentration was measured by infraredabsorption spectrometry (measurement of absorption by OH group for 1.38μm). In addition, quantitative analysis of metallic impurities (Mg, Ca,Ti, Cr, Fe, Ni, Cu, Zn, Co, Mn, Na and K) in the test pieces wasconducted by inductively coupled plasma spectrometry to find that theconcentrations of them were lower than 20 ppb, respectively.

Scattering loss amount of each of the test pieces thus obtained withrespect to ArF excimer laser beam was measured. The results are shown inTable 3. As is clear from Table 3, the silica glasses of the presentinvention (Examples 1-14) satisfied the desired conditions on thescattering loss amount.

Furthermore, as is clear from FIG. 13, when the OH group concentrationwas 1000 ppm or more, the scattering loss amount extremely decreased byreducing the structure determination temperature to 1200 K or lower.

Also, the scattering loss characteristic, polarization characteristic,and birefringence characteristic of each silica glass obtained by theExamples exhibited a center symmetry. Their birefringence amount was 2nm/cm or less.

Further, the results of the measurement of various characteristics ofthe silica glass obtained by the Examples are as indicated in thefollowing. Namely, the internal absorptivity of the above-mentionedsilica glass with a thickness of 10 mm was 0.2%/cm or less with respectto ArF excimer laser. The internal transmittance of the above-mentionedsilica glass with a thickness of 10 mm was 99.8% or more with respect toArF excimer laser. Also, after being irradiated with 1×10⁶ pulses of KrFexcimer laser at an average one-pulse energy density of 400 mJ/cm², theinternal transmittance of the above-mentioned silica glass having athickness of 10 mm was 99.5% or more with respect to light having awavelength of 248 nm. Further, after being irradiated with 1×10⁶ pulsesof ArF excimer laser at an average one-pulse energy density of 100mJ/cm², the internal transmittance of the above-mentioned silica glasshaving a thickness of 10 mm was 99.5% or more with respect to lighthaving a wavelength of 193 nm.

COMPARATIVE EXAMPLE 11

A test piece of silica glass was prepared in the same manner as Example4 except that the holding temperature was set at 1,123 K. Since thestructure was not relaxed during the holding time, the structuredetermination temperature did not become 1,200 K or lower. Also, due toinsufficient annealing, strain was not removed.

COMPARATIVE EXAMPLE 12

A silica glass simply satisfying a specification of a lens materialcharacteristic of Δn≦2×10⁻⁶, a birefringence amount ≦2 nm/cm, and aninternal transmittance of 99.6% or more was used to prepare a projectionlens for an ArF excimer laser stepper. The resolution (L/S) of thusprepared lens was 0.30 μm with respect to the designed L/S of 0.20 μm.Also, its contrast was so unfavorable that the designed property couldnot be obtained. Thus, it was found that the selection of the opticalmember according to such a specification alone was insufficient. It isassumed that, since the absorption loss amount or scattering loss amountexceeded 0.2%/cm, the internal heating within the lens caused by opticalabsorption and the flare generated by optical scattering becameremarkably influential to the deterioration of L/S.

Here, L/S is an abbreviation of “line and space” which is a valuegenerally used as an index for evaluating properties of semiconductormanufacturing.

Homogeneity was measured by oil-on-plate technique in which a He-Nelaser interferometer was used, whereas birefringence was measured byrotational analyzer technique. Internal transmittance was measured by anormal spectrophotometer.

EXAMPLE 15

A silica glass of the present invention satisfying a specification of alens material characteristic of Δn≦2×10⁻⁶ and a birefringence amount ≦2nm/cm as well as both scattering loss amount and absorption loss amountof 0.2%/cm or less was used to prepare a projection lens for ArF excimerlaser stepper. The resolution (L/S) of thus prepared lens was 0.20 μmwith respect to the designed L/S of 0.20 μm. Also, its contrast wasfavorable. Thus, by selecting the optical member according to thisspecification, properties approximating the designed values wereobtained.

Homogeneity was measured by oil-on-plate technique in which a He—Nelaser interferometer was used, whereas birefringence was measured byphase modulation technique. The silica glass used here exhibited a 10-mminternal transmittance exceeding 99.6% at 193 nm.

Also, after being irradiated with 1×10⁶ pulses of ArF excimer laser atan average one-pulse energy density of 100 mJ/cm², the internaltransmittance of the silica glass having a thickness of 10 mm exceeded99.5% with respect to light having a wavelength of 193 nm.

Further, it was confirmed that, after being irradiated with 1×10⁶ pulsesof KrF excimer laser at an average one-pulse energy density of 400mJ/cm², the internal transmittance of the silica glass having athickness of 10 mm exceeded 99.5% with respect to light having awavelength of 248 nm.

When the lens designed is made for a KrF excimer laser, this opticalmember can be used for KrF excimer laser steppers.

A projection lens made of this optical member has a hydrogenconcentration of 5×10 molecules/cm or more with a higher concentrationat the center portion than the periphery.

This projection lens can be used in manufacturing lines for 256-MB VLSI.

As explained in the foregoing, the present invention can provide asilica glass in which the influence of flare and ghost caused by opticalscattering is reduced so as to yield optical properties approximatingthe designed resolution defined at the time of designing a lens, andthus, which achieves high resolution. Further, the present invention canprovide an optical member which includes this silica glass of thepresent invention, and thus, which achieves a favorable contrast. Also,it would be effective in improving throughput.

Therefore, the optical member including the silica glass of the presentinvention can be applied to projection lenses used for any of i-line,ArF, and KrF excimer laser steppers using light in the wavelength regionof 400 nm or shorter. Also, in accordance with the present invention,the performance including resolution and stability of thephotolithography apparatuses can be improved.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims.

The basic Japanese Application Nos. 000479/1995 (7-479) filed on Jan. 6,1995 and 004077/1995 (7-4077) filed on Jan. 13, 1995 are herebyincorporated by reference.

What is claimed is:
 1. An exposure apparatus using light in a wavelengthregion of 400 nm or shorter as exposure light, which comprises: a stageallowing a photosensitive substrate to be held on a main surfacethereof; an illumination optical system for emitting the exposure lightof a wavelength and transferring a pattern of a mask onto saidsubstrate; a projection optical system provided between a surface onwhich the mask is disposed and said substrate, for projecting an imageof the pattern of said mask onto said substrate; and an optical membercomprising the silica glass having a structure determination temperatureof 1,200 K or lower and an OH group concentration of at least 1,000 ppm.2. An exposure apparatus according to claim 1, wherein said silica glasshas a fluorine concentration of at least 300 ppm.
 3. An exposureapparatus according to claim 1, wherein said illumination optical systemcomprises said optical member.
 4. An exposure apparatus according toclaim 1, wherein said projection optical system comprises said opticalmember.
 5. A method for producing a silica glass having a structuredetermination temperature of 1,200 K or lower and an OH groupconcentration of at least 1,000 ppm, said method comprising the stepsof: heating a silica glass having an OH group concentration of 1,000 ppmor more to a temperature of 1,200 K to 1,350 K; maintaining said silicaglass at said temperature for a period of time; and then cooling saidsilica glass to a temperature of 1,000 K or lower at atemperature-lowering rate of 50 K/hr or less to anneal said silicaglass.
 6. A method according to claim 5, further comprising a step ofhydrolyzing a silicon compound in an oxy-hydrogen flame to obtain fineglass particles, and depositing and melting said fine glass particles toobtain the silica glass having an OH group concentration of 1,000 ppm ormore.
 7. A method according to claim 6,wherein a volume ratio of oxygengas to hydrogen gas in said flame is 0.4 or more.
 8. A method accordingto claim 8, further comprising the steps of: hydrolyzing a siliconcompound in an oxy-hydrogen flame to obtain fine glass particles, anddepositing and melting said fine glass particles to obtain a silicaglass having an OH group concentration of 1,000 ppm or more; and thencooling said silica glass from a temperature of at least 1,373 K to atemperature not higher than 1,073 K at a temperature-lowering rate of 50K/hr or less to pre-anneal said silica glass.
 9. A method according toclaim 8, wherein a volume ratio of oxygen gas to hydrogen gas in saidflame is 0.4 or more.